Hmm...your profile says you're a UA Premier Exec.

One idea -- if you fly to/from CDG a lot, could you stick with *A carriers for the next few months? That would give you *A Lounge Access for free as you're *A Gold. Then, once CO joins Star, they should honor *A Gold privileges, so you could fly CO and use CDG *A lounges.

Then you wouldn't need to fork out for that credit card unless you need lounge access domestically on domestic-only flights.